Conventional wind turbines (“turbines”) used for power generation generally have two to five open blades arranged like a propeller, commonly called a rotor, that are mounted perpendicular to the ground. The rotor is mounted to a horizontal shaft attached to a gear box which drives a power generator. The gear box, generator, and other components are contained within a shell typically called a nacelle.
In the most general sense, turbines contain three main components: (1) the base or foundation which anchors the wind turbine in place, (2) the tower which provides the necessary height, and (3) the elevated portion of the wind turbine which includes the rotor, nacelle, and power generating components.
Conventional wind turbines typically require a supporting tower ranging from 60 to 90 meters in height. Taller towers enable larger blades, but also lift the blades into winds that are stronger and more consistent at higher elevations. As a result, the rotor, nacelle, and other components are typically mounted to the top of the tower.
A shrouded wind turbine has a fixed aerodynamic shroud that surrounds the rotor and accelerates air flow through the turbine. This arrangement improves the power delivered from the turbine and its efficiency.
When wind turbines are exposed to excessively high speed winds, the additional wind energy can damage the elevated portion of the wind turbine or support structures. It is not uncommon for wind speeds to become violent enough to damage turbine support structures or even destroy turbines entirely. Even though these events are not frequent, it is an economic and safety imperative for a turbine operator that the wind turbines be constructed to withstand the power of these winds. This results in a turbine that is built with a lot of reinforcing material that is not needed under most operating conditions.
There are a number of prior art attempts to protect wind turbines from high wind damage. For example, in strong wind conditions, the blades of a wind turbine can be furled or the entire turbine can be yawed away from the direction of the wind. These methods work well for protecting the turbine at wind speeds of 45-50 miles per hour. However, in higher wind situations, such as hurricane force winds of 74 mph or higher, damage to the turbine or its structures may still occur. In addition, because the blades of a turbine are located so close to the tower, strong winds are known to bend the blades back so that the blades hit the tower. This phenomenon is known as a tower strike.
The problems of high wind speeds are particularly compounded with certain wind turbine designs. For example, in the case of a shrouded wind turbine, the shroud is a large fixed structure at the top of the turbine tower and thus represents a major source of wind loading, both in total wind force as well as foundation overturning moment. While the use of high towers is ideal for generating electricity, they become a major problem during storms, especially for a shrouded turbine.
Ideally, a wind turbine, or at least the rotors, shroud, and nacelle, would be moved closer to the ground during a major storm. The benefits of lowering a turbine or a significant part of the turbine are twofold, i.e. the rotors, shroud, and nacelle are exposed to less wind by being close to the ground, and the foundation is exposed to less overturning moment because the wind force is not being applied at the end of a long lever.
Some smaller wind turbines, especially those in locations with very strong storms and hurricanes, are designed so that they can lower or fold down during a major storm. However, these defensive operations typically require an operator to go to each individual turbine and operate some equipment to lower the turbine. After the storm, the operator must again go to each individual turbine to restore each turbine in a wind farm into an operational configuration. These defensive operations are more difficult or impossible if there is no electricity on the site.
It would be desirable to provide different methods by which a wind turbine can be protected from excessive wind speeds. Specifically, it would be a significant operational advantage and cost savings if a wind turbine, especially a shrouded turbine, could be protected against a storm without any external user intervention or power, and then restore itself to operational status once the storm has passed.